Procedures are widely known for making glass tubes, for example, by drawing a melt from its free surface. Such procedures enable mechanized fabrication of glass tubes in a rather broad range of size.
One technique of production of glass tubes comprises the following major steps: batch mixing, glass melting, glass tube forming followed by annealing and cooling, breaking-off (cutting-off), annealing the tubes produced in annealing devices and cutting them to specified lengths.
The forming of glass tubes with subsequent annealing and cooling as well as the breaking-off (cutting-off) of the tube are effected in upright drawing plants comprising a drawing compartment wherein the tube is formed, and a drawing pit equipped with guide rolls adapted to draw the glass tube from the free surface of the melt.
The drawing compartment, which is about 2 meters in diameter is heated by natural gas with the help of tangential burners. The compartment is open to the working end of a glass-melting furnace through a wide throat along which the melt is continually admitted into the compartment.
At the center of the compartment, there is a metal pipe through which air is blown inside the tube being formed to provide the manufacture of tubes of specified dimensions. From the exterior, a tube of requisite size is formed with the help of a water cooler which is free to move vertically with respect to the surface of the melt.
The dimensions of the tube being made are adjusted by varying the amount of air admitted into the forming section, the spacing between the cooler and the surface of the melt and the drawing speed.
Upon forming, the tube is transferred to the drawing pit wherein annealing is performed in a certain temperature range, depending on a given chemical composition of the glass, and subsequent cooling to a temperature at which the tube breaks off.
However, upon leaving the upright drawing plant, the tubes are characterized by inadequate mechanical strength and heat resistance which are necessary for reliability. These characteristics result from spontaneous irregular annealing to which the tube may be subjected. Since each tube is formed at a pressure created by blasting the air inside the tube being formed, the air is in a parallel flow with the direction of drawing of the glass tube. In this case, the air is heated in the forming section to such a temperature that it does not cool the tube in the annealing range and the remaining portion of the pit. As each glass tube is cooled in the annealing section only from its exterior, large tensile stresses are generated on the tube internal surface, an additional annealing of the tubes in special annealing outfits becoming therefore imperative.
A reduction in the tensile stresses resulting from repeated heat treatment in the above-mentioned annealing outfits tends to improve mechanical and heat-resisting characteristics of the glass tubes.
It should be noted that most reasonable in terms of the strength characteristics of the tube is distribution of the stresses along the tube thickness such that both its internal and external surfaces are in a compressed state. However, present-day techniques do not insure such stress distribution at which the tubes may have enhanced mechanical and heat-resisting properties.
Additional annealing of glass tubes complicates the production techniques and requires large floor areas for arranging the annealing outfits. Moreover, carrying the glass tubes to and within the annealing plants results in breakage thereby diminishing the yield of finished products.